Process for the production of fluorine

ABSTRACT

THIS INVENTION RELATES TO A PROCESS FOR THE PRODUCTION OF FLUORINE CONTAINING LESS THAN 6% HF BY VOLUME WHICH COMPRISES SUBJECTING TO ELECTROLYSIS AN ANHYDROUS MIXTURE OF NH4F AND HF.

United States Patent O US. Cl. 20459 R 13 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a process for the production of fluorine containing less than 6% HF by volume which comprises subjecting to electrolysis an anhydrous mixture of NH F and HF.

BACKGROUND OF THE INVENTION Fluorine is usually obtained on an industrial scale by the electrolytic decomposition, in the absence of air, of a bath of anhydrous HF added to an alkaline fluoride salt such as KF in order to ionize the bath. The molar composition of this bath is approximately 1 mole KF to 2 moles HF and the temperature at which electrolysis is carried out is not lower than 80 C. Fluorine is recovered at the anode which contains at least 6% HF vapor. The anodic current density does not exceed 15 a./dm. at an average total voltage of volts at the terminals of the electrolytic apparatus. The electrolytic cells and the cathodes are ordinarily made from mild steel. The anodes are made from carbon. The temperature of the electrolytic bath is maintained by circulation of cold water through a jacket surrounding the cell and a centrally positioned coil. The lifespan of the cells is limited to the duration of the anodes which eventually crack from localized heating and chemical corrosion.

Other studies have involved the electrolysis of mixtures of hydrofluoric acid and anhydrous ammonia, or what amounts to the same, hydrofluoric acid and anhydrous ammonium fluoride.

Gmelins Hanbuch fur der anorganische Chemie published in 1959 reports the results of the research work of Otto Ruff on the electrolysis of baths containing NH and HF. The composition of the gaseous mixture recovered at the anode is found to be dependent upon the concentration of NH in the electrolytic bath; if the concentration of NH is high, mostly nitrogen, is produced but if the concentration of NH is low, mostly fluorine is produced. A table at page 238 of this publication reports the products obtained from the electrolysis of baths having concentrations of NH ranging from 31% to 16% by weight. At a process temperature of about 125 C. and at an NH concentration of from 31 to 29%, nitrogen was recovered as the principal product. When the NH;, concentration was lowered to about 26% and the process temperature lowered to within the range 112-65" 0., fluorine first began to be produced. In the last range of concentration of NH reported, i.e., 24.5 to 16%, and at a process temperature of from 65 to 40 C., NF was recovered as the principal product along with a number of other products, among them being fluorine.

The results reported in Gmelins Handbuch fur der anorganische Chemie agree with what is stated in The Encyclopedia of Chemistry by Hempel at page 856: With less than 10 percent NH the product of electrolysis is fluorine. Spears and Hackermann report in their paper, The Electrolysis of Ammonium Bifluoride in Anhydrous Hydrogen Fluoride, Journal of the Electrochemical Society, May 1968, vol. 115, No. 5, that nearly pure "ice fluorine was obtained by the electrolysis of a mixture of anhydrous HF and NH F containing less than 2% NH by weight, at a temperature of 20 C. at 5 volts and at a current density not exceeding 0.2 a./dm.

Applicants have unexpectedly found in view of the teaching of the prior art that electrolysis of an anhydrous mixture of NH F and HF under certain advantageous conditions will produce fluorine containing less than 6%, and often less than 3% by volume HF in addition to trace amounts of other products. The fluorine which is produced by the process of this invention is at least equal in quality to the fluorine produced by the known process of electrolysis of baths containing KF and HF and under conditions which are more favorable than hereinbefore possible.

SUMMARY OF THE INVENTION This invention relates to a process for the production of fluorine containing less than 6% HF by volume which comprises subjecting to electrolysis in a conventional electrolytic cell in the absence of air at a voltage of at least 6 volts and within a temperature range of from 0 to 50 C., an anhydrous mixture of NH F and HF characterized by the fact that the content of NH in the bath as calculated from NH F is held substantially between 17.5 and 20% by weight.

DESCRIPTION OF THE PREFERRED EMBODIMENT The process of this invention comprises electrolyzing in the absence of air at a temperature between 0 and 50 C., at at least 6 volts in a steel or Monel metal cell with a steel cathode and a carbon anode, an anhydrous mixture of NH HF and HF continuously or intermittently fed with HF at regular intervals, the mixture characterized in that it contains between 17.5 and 20.5% by weight, and advantageously, about 19% by weight of NH, calculated from NH F, and advantageously, a molar ratio of NH F to HF between 1:3 and 1:2.3. A molar ratio of NH F to HF of 1:2.6 has been found to provide especial- 1y advantageous results.

This process utilizes the unusual and unobvious properties which characterize the above range of concentration of bath components which properties were not known hereinbefore.

According to one embodiment of this process, the NH F component of the bath can be partially replaced with up to about 25% of the molar fraction of an alkaline fluoride. For example, with the molar ratio of 1 mole NH F to 2.6 moles HF, a portion of the NH F can be replaced with an alkaline fluoride salt such as KF to provide a bath having the composition: 0.25 mole KF, 0.75 mole NH F and 2.6 moles HF. When a part of NH F is replaced by an alkaline fluoride, the weight concentration of NH does not remain between 17.5 and 20%. However, the combined molar concentration of NH F and alkaline fluoride remain within the ratio indicated for the molar concentration of NH F and HF when the latter is used alone.

The apparatus which is conventionally employed for the electrolysis of fluorine may be advantageously employed for the process of this invention, i.e., a watercooled cell made of steel or Monel metal and cathodes of steel and anodes of carbon.

The process of this invention results in considerable advantages over known process utilizing baths of 1 mole KF and 2 moles HF above C.

To begin with, the anodic voltage of the instant process is lower by at least 0.5 to 1.0 volts over that of known processes at the same current density. The apparent resistance of the bath employed in the process of this invention is significantly lower than that of the baths of known processes. The efiiciency of the cell is improved. For example, when one carries out the process of this invention at 30 C. at a current density of 15 a./dm. an energy saving of about 25% is realized compared to the known process carried out at 90 C. and at a current density of 15 a./dm. 7 to 8 volts are required in the instant process compared to 9 to 10 volts in the prior art process.

Another advantage of this invention is to be found in the reduction of heat given off by the anodes which reduction is brought about by a lowering of the voltage and reduced resistance of the bath. Thus one can increase current density without abnormal wear of the anode. At a voltage level of from 9 to 10 volts, one can carry out the process of this invention at a temperature of 30 C. and a current density of 35 a./dm. Thus for the same amount of kwh. for kg. of fluorine produced, the instant process results in substantial economies upon deterioration of the cell.

Another advantage of this invention is that at a process temperature as low as to 35 C., the amount of HF vapor in the bath is low thus permitting the recovery of fluorine containing very little HF. Furthermore, in this temperature range, one avoids undesirable secondary chemical reactions between NH and the fluorine being evolved which reactions mainly give rise to N1 Accordingly, one recovers fluorine which does not contain explosive compounds of fluorine and nitrogen such as the fluoroamines. Thus the process of this invention results not only in fluorine having a high degree of purity, but at the same time, avoids the danger of an explosion.

Another advantage of the process of this invention is that the lifespan of the anodes is increased. In the temperature range at which electrolysis is carried out, corrosion due to fluorine is reduced and a plastic material, such as polypropylene, methylpolyrnetacrylate can be used for the cell tank. Moreover, because of lower anodic voltage, heating of the anodes is reduced and the temperature of the anodes does not exceed the temperature of the bath by more than 10 C. while the temperature of the anodes may exceed the temperature of the bath in a known process by more than 30 C. The lower amount of heat and the better thermal conductivity of the bath reduce the thermal gradient within the interior of the anodes and therefore reduce the risk of cracking and deteriorating the contacts with the bars introducing the electric current.

The baths utilized in the process of this invention have melting points between 7 C. to 23 C. while the baths consisting of 1 mole KF22 moles HF do not melt below 72 C. This fact, combined with the better thermal conductivity of the baths of this invention result in easier maintenance of the temperature of the cells which are effectively cooled by circulation of water. The cell walls are not covered with crystals. Slowing or stopping electrolysis generally does not lead to solidification of the bath and restarting electrolysis no longer requires the great inconvenience of melting the contents of the cell which are more easily managed in this invention.

This invention provides a process having greater econonly, reliability and productivity and one which is more easily managed. By comparison to a process employing a bath containing NH F and HF having less than 10% NH by weight as calculated from NH F, the process of this invention provides numerous advantages. In the first place, there is less HP in the baths of the instant invention thereby resulting in a considerable reduction in the corrosion of the metallic surfaces and the carbon anodes. Furthermore, the vapor pressure of the baths of the process of this invention is much lower. Electrolysis of a bath containing less than 10% NH at the usual temperature range produces a vapor pressure over 100 torr whereas our bath which contains 19% NH produces a vapor pressure of only 15 torr. In the latter bath, the amount of HF contained in the fluorine produced will be reduced and therefore, the fluorine recovered from the process of this invention will have a higher degree of purity by comparison, the vapor tension of baths containing 1 mole KF12 moles HF is about 45 torr and the fluorine produced contains 6% HF by volume. The fluorine produced by the process of this invention is purer and therefore economies are realized in the purification apparatus. And in contrast to the 20 C. working temperature reported by Spears and Hackermann, the process of this invention may be carried out at ambient temperature.

The following non-limitative examples illustrate the process of this invention.

Example 1 An electrolytic cell having a single anode was operated.

at a current density of 15 a./dm. for 12000 hours.

The centrally positioned carbon anode had a useful surface area of 7 dm. The steel cell was electrically connected to two steel cathodes situated on both sides of thecarbon anode plate. A flat electrically insulated diaphragm comprising a grating of Monel metal threads was situated in the middle of the distance (40 mm.) which separated the anode and cathodes. Cooling was accomplished by the circulation of water through a jacket surrounding the cell.

The bath used in the above cell contained a mixture of anhydrous NH F and HP in which the concentration of NH in the bath was maintained at about 19.1% by weight by constant feeding of HF and the temperature was held at about 28 C. during the electrolysis. The voltage of the terminals was 7.8 volts at amp. and the concentration of HF in the fluorine produced at the anode was 2.9% by volume.

Example 2 An electrolytic cell having 32 anodes was operated at a current density of 15 a./dm. for 800 hours.

The 32 anodes of this cell were rectangular carbon plates providing a useful surface area of 270 dm. The steel cell was electrically connected to the steel cathodes of which the useful surface was 560 dm. The space between the anodes and cathodes was 40 mm. The electrically insulated Monel metal diaphragm was situated 20 mm. away from the anodes. Cooling was accomplished by the circulation of water through a jacket surrounding the cell and a centrally positioned coil.

The bath used in the above cell, contained anhydrous NH F in which the concentration of NH in the bath was maintained at about 19% by weight of constant feeding of HF and the temperature was held at about 30 C. during the electrolysis. The voltage of the terminals was 7.8 volts at 4050 amp. and the concentration of HP in the fluorine produced at the anode 2.9% by volume.

Example 3 The cell employed in Example 2 was operated at a current density of 22 a./dm. for 1000 hours.

The bath used in the above cell contained a mixture of anhydrous NH F and HF in which the concentration of NH in the bath varied between 18 and 20% by weight. The cooling means provided for the cell was the same as that provided in Example 2. The temperature of the bath was held at about 32 C. during the electrolysis and the voltage of the terminals was 8.9 v. at 6000 amps. The concentration of HP in the anodic fluorine was 3.1% by volume.

Example 4 The cell employed in Example 1 was operated at a current density of 28 a./dm. for 2460 hours.

The bath used in the above cell contained a mixture of anhydrous NH F and HF in which the concentration of NH was maintained at 19.2% by weight and the temperature was held to about 32 C. during the electrolysis. During the operation of the cell at a current strength of 200 amp, the voltage of the terminals constantly remained at 9.3 v. A volumetric analysis of the gas produced at the anode gave the following results:

Percent HF 3 S 3% 2+ 2 CF; -1 Traces F Balance The anode was removed from the cell and carefully inspected. It was observed to be in excellent condition and could be restored to service. The surface of the anode appeared to be intact and the contact between the anode and the bar introducing the electric current was still in good condition. Indeed, by comparison with the new contacts which wasted 13 watts at 200 amp., the worn contact wasted only 50 watts. After an equivalent operating time but with a bath of 1 mole KFz2 moles HF at 90 C. at 100 amp. and a current density of 14.3 a./dm. the used contacts wasted about 600 Watts.

Example An electrolytic cell having four anodes was operated at a current density of 34 a./dm. for 960 hours.

The four carbon anodes of this cell had a useful surface area of 35 dm. The cell was made of steel and the cathodes which were made of steel were electrically connected to the cell. An electrically insulated diaphragm made of Monel metal was provided as in the apparatus of the preceding examples. The distance between the cathode and anodes was about 40 mm. and cooling was accomplished by circulation of water through a jacket surrounding the cell. The bath contained 14% NH by weight, 15% KF by weight and anhydrous HF was fed into the bath at regular intervals to maintain the combined concentrations of NH F and KP to HF between 1:3 and 1:2.3. The voltage at the terminals which carried at 1200 amps. was 910.1 volts and the temperature was held at about 45 C. The gas recovered at the anode had the following volumetric composition:

Percent HF 5.0 NF 0.25 N +O 0.4 CF Traces F Remainder After their removal and inspection, no deterioration of the contacts/anodes/bars was observed and the anodes were in excellent conditions.

We claim:

1. A process for the production of fluorine containing less than about 6% HF by volume which comprises subjecting to electrolysis in the absence of air at a voltage of at least 6 volts and within a temperature range of 6 from about 0 to 50 C., a bath of an anhydrous mixture of NH F and HF, adding HP to the bath during the electrolysis and maintaining the content of NH in the bath as calculated from NH F substantially between about 17.5 and 20.5% by weight, and recovering said fluorine.

2. The process of claim 1 wherein HF is added continuously to the bath during the electrolysis.

3. The process of claim 1 wherein HF is added at periodic intervals to the bath during the electrolysis.

4. The process of claim 1 wherein the temperature of the bath is maintained from about 15 to 35 C.

5. The process of claim 1 wherein the content of NH in the bath as calculated from NH F is about 19%.

6. The process of claim 1 wherein the molar ratio of NH F to HF is substantially within the range from about 1:3 to 1:23

7. The process of claim 6 wherein the molar ratio of NH F to HP is substantially about 1:2.6.

8. The process of claim 1 wherein a portion of NH F not exceeding 25% of its molar proportion is replaced mole for mole with at least one alkaline fluoride.

9. The process of claim 8 in which the alkaline fluoride is potassium fluoride.

10. The process of claim 1 wherein the current density is 15 a./dm. at a voltage of from about 7 to 8 volts.

11. The process of claim 1 wherein the current density is at least 28 a./dm. at a voltage of from about 9 to 10 volts.

12. The process of claim 9 wherein the electrolytic bath has substantially the composition expressed as moles percent: 0.25 KF, 0.75 NH F and 2.6 HF.

13. A process for the production of fluorine containing less than about 6% HF by volume which comprises subjecting to electrolysis in the absence of air at a voltage of at least 6 volts and within a temperature range of from about 0 to 50 C., a bath of an anhydrous mixture of NH F and HF with a portion of NH F not exceeding 25 of its molar proportion replaced mole for mole with at least one alkaline fluoride and the ratio of the combined molar concentration of NH F and KF to HF is substantially maintained during the electrolysis within the range from about 1:3 to 1:2.3, and recovering said fluorine.

References Cited UNITED STATES PATENTS 5/1970 Childs et al. 204-59 8/1964 Davies 204-60 OTHER REFERENCES FREDERICK C. EDMUNDSON, Primary Examiner UNITED STATES PATENT OFFICE CERTIFICATE CORRECTION Patent No. 3 729, 395 Dated Aior'il 21+, 1973 Inventor(s) i l aron, et a1.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column- Lr, line L B "weight of constant" should read weight by constant Column 5, lines 38 and 39 "carried at 1200 amps" should read carried a 1200 amps Column 5, line 11,3 "HF 5.0%" should read HP 5.9%

Signed and sealed this 26th day of March 19714..

(SEAL) Attest:

EDWARD MEETCHERJR. C. MARSHALL DANN Attesting Officer 7 Commissioner of Patents I USCOMM-DC 60376-969 v: u.s. GOVERNMENT PRINTING OFFICE: "89 chass s. Q

FORM PO-105O (IO-69) 

